In semiconductor production, wafers are processed sequentially in a plurality of process steps during the production process. With increasing integration density, requirements in terms of the quality of features configured on the wafer are rising. It is advantageous for this purpose if the quality even of individual process steps, for example lithography steps, can be dependably assessed during the production process and prior to a subsequent process step. The reason is that if a determination can be made, directly after a process step is performed and before final completion of a production process, that a wafer or the features configured on a wafer are defective, the wafer can be immediately discarded with no need to perform other subsequent process steps. Or the wafer identified as defective can be given special additional treatment until satisfactory quality is achieved. Efficiency and yield in semiconductor processing can thus be improved.
Optical apparatuses are particularly suitable for inspecting the surface of wafers. Optical apparatuses are known that can detect, by image recognition, a very wide variety of features on the surface of a wafer, the wafer usually being bright-field illuminated and scanned with a camera (matrix or line camera).
An inspection apparatus of this kind of the KLA-Tencor Corporation is described in the article “Lithography Defects: Reducing and Managing Yield Killers through Photo Cell Monitoring,” by Ingrid Peterson, Gay Thompson, Tony DiBiase, and Scott Ashkenaz, Spring 2000, Yield Management Solutions. The wafer inspection apparatus described therein works with an incident-light illumination device that examines microdefects with low contrast using a bright-field/dark-field illumination system.
U.S. Pat. No. 4,772,126 discloses an apparatus and a method for the detection of particles on the surface of a wafer. The wafer is held rotatably on a wafer receiving device. Two laser beams are incident, mutually orthogonally in projection onto the surface of the wafer, onto the surface of the wafer at a raking incidence angle. Scattered light that derives from defects and particles on the surface of the wafer in the illuminated region is imaged, in a dark-field arrangement, into an image capture device oriented perpendicular to the surface of the wafer, and subsequently evaluated. The laser beams are imaged onto the surface of the wafer by means of lenses that are arranged at the circumferential edge of the wafer. A certain minimum spacing between the lens and the circumferential edge of the wafer must be maintained; this considerably limits imaging capabilities.
U.S. Pat. No. 6,292,260 B1 discloses an apparatus and a method for optical inspection of surface features on a wafer. Two laser beams are incident oppositely and at a raking incidence angle onto the surface of a wafer, a projection of the illuminating light beams onto the surface of the wafer enclosing an angle of 45° with the respective linear features extending on the surface of the wafer. The laser light sources are arranged directly at the circumferential edge of the wafer, which increases the base area of the inspection apparatus and is disadvantageous in terms of performing contamination-creating maintenance work on the laser light sources.
WO 99/02977 discloses an apparatus and a method for inspection of the surface of a wafer, one or two laser beams being incident at 45°, in projection onto the surface of the wafer, with respect to the features on the wafer. Light reflected directly from the surface of the wafer, and diffraction orders that derive from diffraction of the incident light at the features on the surface of the wafer, are detected in spectrally resolved fashion in a bright-field arrangement using an image capture device, and evaluated.
With the aforementioned inspection apparatuses, the variation capabilities in terms of imaging the illuminating light beams onto the surface of the wafer are limited. It is additionally disadvantageous that the adjustment of light sources and imaging optical elements that are arranged in the vicinity of the circumferential edge of the wafer is difficult, and can result in damage to the wafer surface.